Abstract
Silk cocoons obtained from silkworms are the primary source of commercial silk, making the silkworm an economically important insect. However, the silk industry suffers significant losses due to various virus infections. Bombyx mori bidensovirus (BmBDV) is one of the pathogens that cause flacherie disease in silkworms. Most silkworm strains die after BmBDV infection. However, certain silkworm strains show resistance to the virus, which is determined by a single recessive gene, nsd-2. The +nsd-2 gene (allele of nsd-2; the susceptibility gene) encodes a putative amino acid transporter expressed only in the insect’s midgut, where BmBDV can infect, suggesting that this membrane protein may function as a receptor for BmBDV. Interestingly, the expression analysis revealed no changes in the +nsd-2 gene expression levels in virus-uninfected silkworms, whereas the gene expression drastically decreased in the virus-infected silkworm. This condition indicates that the host factor’s expression, the putative virus receptor, is affected by BmBDV infection. It has recently been reported that the expression levels of some host genes encoding cuticle, antioxidant, and immune response-related proteins were significantly regulated by BmBDV infection. In this review, we discuss the host response against virus infection based on our knowledge and long-term research experience in this field.
Highlights
Bombyx mori has been bred in captivity for around 5000 years, and it is a completely domesticated species of the silkworm [1]
We examined the expression level of genes encoding other transporters in the midgut after virus infection [26] to investigate whether the decrease in expression levels caused by virus propagation is exhibited only by the +nsd-2 gene
The virus isolate has been found in Japan, China, and Indian isolate (India), and their characteristics and pathogenicity to silkworms have been studied so far [10,11,12]
Summary
Bombyx mori has been bred in captivity for around 5000 years, and it is a completely domesticated species of the silkworm [1]. Many studies have been carried out to elucidate the interaction between the silkworm and the viruses, and a variety of molecules and pathways involved in the silkworm’s immune responses to viruses, such as antimicrobial peptides, prophenoloxidase-activating system, apoptosis, reactive oxygen species (ROS), small RNA, and related molecules, have been identified [2]. Considering this background, we focused on the DNV’s infection mechanism and host responses to the virus, and we were successful in isolating a vital host factor (putative virus receptor) for the first time in the insect virus. We would like to discuss the following items: (i) the silkworm’s resistance mechanism against the DNV infection; (ii) the relationships between a putative virus receptor and DNV infection; (iii) the host genes whose expression levels fluctuate due to DNV infection
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